1. Field of the Invention
The present invention pertains to a method for etching a trench in a silicon substrate. In particular, the present invention pertains to particular plasma etch chemistries, process conditions, and a series of steps which may be used to produce rounded top corners on a trench etched in a silicon substrate. The term "top trench corner" refers to the transition from the upper sidewall of a trench to the upper surface of the substrate in which the trench is formed.
2. Brief Description of the Background Art
Trenches formed in silicon using traditional etching methods typically have sharp, squared-off top corners. These sharp, squared-off corners lead to high field stress in film layers subsequently deposited thereon during further processing steps. The high field stress can potentially lead to the electrical breakdown of the overlying deposited film layers. Further, the sharp, squared-off corners are a point of charge accumulation, which can cause edge leakage. Rounding of the top corners in trench structures can be critical for device performance. However, rounding of the corners in a manner which results in a loss of device active area is undesirable.
Various methods for obtaining a rounded top corner on a trench formed in a silicon substrate are known in the art. For example, U.S. Pat. No. 5,843,846, issued Dec. 1, 1998 to Nguyen et al., discloses a method for rounding the top corners of a sub-micron trench in a semiconductor device after trench formation. The method comprises exposing the previously formed trench to a gas comprising a carbon-fluorine gas, argon, and nitrogen directly after trench formation. The combination of the carbon-fluorine and nitrogen gases etch back the silicon nitride and stress relief oxide layers in order to expose the top corners of the trench. As the top corners of the substrate are exposed, nitrogen and argon gases are said to sputter the top corners, rounding them as the etch process completes the trench. The method is preferably performed using a low density parallel plate etch reactor.
Commonly assigned, copending U.S. application Ser. No. 09/042,249, filed Mar. 13, 1998, discloses a method of obtaining a rounded top corner on a trench formed in a semiconductor substrate comprising the following steps: (a) providing a film stack comprising the following layers, from the upper surface of the film stack toward the underlying substrate, (I) a first layer of patterned material (typically, a patterned photoresist) which is resistant to a wet etch solution used to etch an underlying second layer and which is resistant to dry etch components used to etch the semiconductor substrate (typically, silicon), and (ii) a second layer of material (typically, silicon dioxide) which can be preferentially etched using a wet etch solution, wherein the second layer of material is deposited directly on top of the semiconductor substrate; (b) wet etching the second layer by immersing the film stack in a wet etch solution for a period of time sufficient to form an undercut beneath the first layer and to expose the underlying semiconductor substrate; and c) isotropically dry etching the exposed semiconductor substrate so as to form a trench in the semiconductor substrate.
A sharp corner at the bottom of a trench can also be a source of stress, causing problems of the kind described with reference to the top corners of the trench. In addition, a rounded corner facilitates filling of the trench with a reduced possibility of trapping voids within the fill material. It is desirable to provide an etch process which provides a rounded bottom corner while maintaining a desired trench sidewall angle, for example, about 80.degree. to 90.degree..